Crude wet process phosphoric acid is prepared by the acid digestion of phosphate rock. Commonly, the crude phosphoric acid contains a wide variety of cationic impurities such as calcium, magnesium, aluminum, iron and arsenic. For industrial applications such as food products it is desirable to remove the major portion of these impurities.
It is known to purify phosphoric acid by solvent extraction techniques. In solvent extraction procedures, the crude phosphoric acid is contacted with an organic solvent of limited water miscibility. The organic solvent displays a greater preference for phosphoric acid than it does for the acid contained impurities and effects a selective separation. Unfortunately, it is typical for five to fifteen percent of the potential phosphoric acid (P.sub.2 O.sub.5) value of the crude acid to be tied-up as metal salts. These salts are not preferentially extracted by the organic solvent.
The conventional method of recovering the phosphoric acid values of crude acid contained metal salts is exemplified by U.S. Pat. Nos. 3,556,739 and 3,595,613. These patents teach that addition of strong acids such as hydrochloric acid or sulfuric acid to the crude acid will promote transfer of additional phosphoric acid values into the organic solvent phase of an extraction system. Hydrochloric acid addition introduces the potential problem of chloride ion removal. Sulfuric acid addition is preferred to the use of hydrochloric acid because of its generally lower cost and greater availability. Nevertheless, sulfuric acid has the attendant disadvantage that it forms precipitates when contacted with the crude phosphoric acid. These precipitated sulfates promote fouling of extraction equipment.